The present invention relates to charge-coupled devices and in particular to a charge-coupled device cell employing a channel potential trench to improve charge transfer efficiency.
A charge-coupled device (CCD) includes an array of closely spaced cells aligned along a lateral path. Each cell includes an electrode formed on an oxide layer covering a semiconductor substrate having a channel region for storing charge carriers. When clock signals of differing phase drive electrodes of neighboring cells, an electrical field develops between channel regions of neighboring cells, and this electrical field drives charge carriers stored in the channel region of one cell into the channel region of its neighboring cell. With appropriately phased clock signals applied to electrodes of neighboring CCD cells, charges shift laterally from cell-to-cell.
During initial stages of charge transfer between adjacent CCD cells, the clock-induced potential gradient between the channel regions of the cells provides a strong electrical field driving carriers quickly into the receiving cell. A high carrier concentration gradient between channel regions of the adjacent cells also encourages carrier flow by diffusion. However, the carrier concentration gradient at the edge of the transferring cell decreases as fewer and fewer carriers are left. Meanwhile, the Fermi level under the transferring cell also decreases with decreasing carrier density. This leads to reduced potential gradient across the junction between the cells, thereby reducing carrier flow rate at the junction.
The "charge transfer efficiency" of a charge-coupled device is the ratio of charge transferred out of a cell by the end of a charge transfer cycle to the initial charge stored in the cell at the beginning of the transfer cycle. A high charge transfer efficiency prevents substantial degradation of charge passing through the charge-coupled device. As the frequency of clock signals applied to the electrodes of CCD cells increases, the time available for all charge carriers to move from one cell to its neighboring cell decreases. At high clock frequencies, a substantial portion of the charge may remain behind at the end of a transfer cycle. Thus as the frequency of operation of a charge-coupled device increases, charge transfer efficiency decreases.
A conventional two-phase CCD cell, as described, for example, in U.S. Pat. No. 3,986,198 entitled "Introducing Signal at Low Noise Level to Charge-Coupled Circuit" issued Oct. 12, 1976 to Kosonocky, varies dopant concentration in the channel to provide a two-tiered built-in channel potential. A lower potential tier forms a potential well in the cell for storing charge. A higher potential tier between the potential well of the cell and a potential well of its preceding neighbor cell acts as a potential barrier to prevent backflow of charge from well-to-well.
During initial stages of charge transfer, carriers in the well of one cell are at a substantially higher potential than the potential barrier of the next succeeding cell, and the carriers rapidly drift and diffuse over the barrier into the well of the next cell. However, during later stages of charge transfer when only a few charge carriers remain in the potential well of the cell, the electrical field and the concentration gradient at the cell junction grow small and the remaining few carriers move through the well and into the next cell primarily limited by a diffusion process at a relatively slow rate that is a function of the length of the well.
In U.S. Pat. No. 3,796,932 entitled "Charge Coupled Devices Employing Nonuniform Concentrations of Immobile Charge along the Information Channel", issued Mar. 12, 1974 to Amelio et al, the channel region in each CCD cell has a tilted built-in potential gradient in the lateral direction. The tilted potential gradient provides an electrical field on charge carriers within the channel region to increase the rate of charge carrier drift through the channel region. The drift rate increase is most apparent during the later portion of a clock signal phase when carrier concentration and clock-induced potential gradients at the junction between neighboring cells are small.
While a tilted channel potential gradient improves charge transfer efficiency at higher clock frequencies, a tilted channel CCD cell is more expensive to manufacture than the two-tier cell, and charge transfer rate is substantially improved only at relatively low charge levels.